Internet giants Google and Twitter have reactivated their voice-to-tweet service to combat an ongoing communications blackout in Syria.

Internet and telephone lines in much of the country are down for a second consecutive day, amid renewed fighting between rebels and government troops in southern Damascus.

Government officials say "terrorists" have attacked internet lines, but opposition and human-rights groups are blaming the Syrian government for the outage.

It has previously cut off access during major operations, but a nation-wide shutdown is unprecedented.

In response, Google and Twitter announced they reactivated the voice-to-tweet program to allow Syrians affected by the shutdown to get messages out.

The service allows people with a telephone connection to send a tweet by speaking on their phones.

Google and Twitter say the system was used last year when the internet was shut down by authorities in Egypt for several days.

Anonymous, a global hacking group that opposes internet censorship, has threatened to shut down Syrian government websites around the world in response to the blackout.

"As we discovered in Egypt, where the dictator [Hosni] Mubarak did something similar - this is not damage that can be easily or quickly repaired," it said in a statement, referring to an internet outage during the early days of the 2011 uprising in Egypt.

French foreign ministry spokesman Philippe Lalliot said the communications cut was a matter of "extreme concern".

"It is another demonstration of what the Damascus regime is doing to hold its people hostage. We call on the Damascus regime to re-establish communications without delay," he said.

CloudFlare, a firm that helps accelerate internet traffic, said on its blog that saboteurs would have had to simultaneously cut three undersea cables into the Mediterranean city of Tartous and also an overland cable through Turkey in order to cut off the entire country's internet access.

"That is unlikely to have happened," it said.

Flights cancelled

Many flights to and out of the Syrian capital have been cancelled for a second day after clashes spread to the road leading to the city's airport. Several airlines, including EgyptAir and Emirates - say they are not able to use Damascus airport.

Activists said security forces clashed with rebels trying to topple president Bashar al-Assad around the Aqraba and Babilla districts on the south-eastern outskirts of the road that lead to the international airport.

A resident of central Damascus said he saw black smoke rising from the east and the south of the city on Friday morning and could hear the constant boom of shelling. State television said government forces were fighting rebels in those areas.

An aviation source in neighbouring Jordan said two Syrian Air flights crossed Jordanian air space heading for the Syrian capital on Friday evening and that Damascus airport was open, although international airlines were staying away.

The head of the national airline, Syria Air, said services were operating according to schedule, state television reported.

EgyptAir and Emirates have suspended flights to Damascus in response to the recent violence and there was no sign that Air Arabia and flydubai had flown scheduled trips on Friday.

"Airlines are not operating to Damascus today," said a Dubai-based airline official.

The Syrian Observatory for Human Rights, a British-based opposition monitoring group, said jets were bombarding targets in rural areas around Aqraba and Babilla, where rebels clashed with president Bashar al-Assad's forces.

The Observatory's director, Rami Abdelrahman, said the airport road was open, but there was minimal traffic.

'Decisive phase'

A rebel told Reuters that he was on the airport road and that his fighters would not let the airport operate.

"We will never open the road. It's still closed and it will remain closed. We will not allow planes to arrive," he said.

Rebels said that at least one mortar round was fired at the airport during clashes on Thursday.

"We want to liberate the airport because of reports we see and our own information we have that shows civilian airplanes are being flown in here with weapons for the regime. It is our right to stop this," rebel spokesman Musaab Abu Qitada said.

US and European officials said rebels were making gains in Syria, gradually eroding Mr Assad's power, but said the fighting had not yet shifted completely in their favour.

A Damascus-based diplomat said he believed the escalation in fighting around the capital was part of a government offensive which aimed to seal off the state-controlled centre of the city from rebel-held rural areas to the south and east.

Activists say Mr Assad's forces have also been shelling the Daraya district to the south-west of the city, trying to prevent rebels from cementing their hold of an area which could give them a presence in a continuous arc from the north-east to south-west of the capital's outer districts.

"I don't know whether the shelling has succeeded in pushing back the FSA [rebels] - experience shows that they return very quickly anyway," the diplomat said.